Not only did juvenile Fraser sockeye host higher lice levels in the Georgia Strait after they passed salmon farms, these fish hosted an order of magnitude more sea lice than Skeena and Nass River sockeye that migrated along the north coast where there are no farms.

The study also recorded the highest lice levels on juvenile sockeye near a farmed salmon processing plant in the Georgia Strait, heightening concern for the full potential impact of the salmon farm industry on wild salmon in this region.

Importantly, researchers of this study were the last to see juvenile Fraser sockeye alive before they left for the open ocean in 2007 and failed to return two years later; the collapse of Fraser sockeye in 2009 is now the topic of a Canadian federal inquiry. Results of this study show that juvenile sockeye were heavily infected with sea lice after they passed salmon farms that fateful year, and lice are a possible clue to their disappearance.

But do we know whether sea lice cause specific harm to juvenile sockeye from Canada's jewel of salmon rivers, the Fraser?

There has been no shortage of scientific papers published in peer-reviewed journals over the last decade linking salmon farms to sea lice infestations of wild juvenile salmon. Of these, several papers have documented direct and indirect mortality effects, and overall declines in wild salmon populations.

Although we know little about how sea lice may affect juvenile sockeye, in addition to any direct physical and behavioural impacts sea lice can serve as vectors of disease or indicators of other farm-origin pathogens. Given the high intensities of lice observed on some juveniles in this study -- up to 28 lice/fish -- there's an urgent need to understand the extent of threat posed by sea lice to juvenile Fraser River sockeye.

Let's not wait another decade to find out what we've learned about wild Atlantic salmon, sea trout, Pacific pink, chum, and coho salmon: that sea lice infestations from open net-pen salmon farms have been linked with subsequent declines in wild populations.

It is clear that sea lice from salmon farms are yet another stressor for sockeye already subjected to multiple human impacts. Notably, risks to juvenile sockeye from open net-pen salmon farms can be much more easily mitigated than changes to ocean conditions from climate change and ocean acidification. In terms of salmon conservation, efforts to protect these fish will be most effective when the stressors that are the easiest to alleviate are targeted first.

This article was co-authored by Michael Price, Raincoast Conservation Foundation biologist and lead author of the PLoS ONE paper.